1. Field of Invention
The present invention relates to a family of glasses that absorb ultraviolet (UV) radiation and filter out yellow light in the visible region of the spectrum.
2. Description of Related Art
It is well known that ultraviolet (UV) radiation not only has harmful effects on humans, but can also cause degradation and discoloration in such items as paints, fabrics, and plastics. UV radiation covers wavelengths ranging from 60 nm-380 nm. The sun is not the only light source that emits UV waves. Various types of artificial lighting, such as halogen lamps and especially discharge and arc lamps, may also emit UV radiation. Accordingly, there is an interest in minimizing UV radiation emitted by artificial light sources as well. This may be achieved, for example, by utilizing UV absorbing glass in the fabrication of lamp envelopes, reflectors and lenses.
Equally as important as the interest in minimizing UV radiation emitted by artificial light source is the interest in producing lighting devices having improved illumination and high color rendering. This interest is pursued especially zealously in the automotive industry where there is a push for vehicle headlight systems that can provide improved night visibility. One option is high-intensity discharge (HID) lamps which are usually found on luxury and high-end performance automobiles. Although, HID lamps offer improved brightness and night visibility, they are costly. Therefore, there exists an interest in providing less; expensive lamps with similar performance properties. One method of achieving this objective is to approximate the illumination properties of HID lamps in less expensive light sources, such as tungsten halogen lamps or incandescent lamps. This may be achieved by employing lamp envelopes or lenses made of glass which filters visible light, for example neodymium containing glass which raises the color temperature of halogen or incandescent light sources. This results in a light source having improved brightness which can provide higher contrast definition to the illuminated objects.
Neodymium containing glass is known in the glass industry, and has historically found employment in the aviation and navigation fields..sup.1 Neodymium, a rare-earth element, is most often incorporated in glass as an oxide, more specifically Nd.sub.2 O.sub.3. Long known as a coloring agent, neodymium like the other rare-earth elements possesses an absorption spectra that extends over both the visible and invisible regions, transferring practically unchanged to base compounds, such as glasses..sup.2 Neodymium absorbs light in the yellow region of the visible spectrum, between 568 and 590 nm. As a result, light passing through neodymium containing glass accentuates the red and green tones in the surrounding environment..sup.3 Experiments have also shown that neodymium containing glass provides an increase in visibility during foggy weather..sup.4
Recently, neodymium containing glass has been proposed for the production of automobile headlights to reduce the visual discomfort from oncoming cars (U.S. Pat. No. 5,548,491) and for reflective lamps to yield a high color rendering (U.S. Pat. No. 4,315,186). For example in U.S. Pat. No. 5,548,491, Karpen discloses a motor vehicle headlight, either an incandescent lamp or a tungsten-halogen lamp comprising glass bulbs produced from neodymium doped soda lime glass and borosilicate or quartz glass, respectively. Neodymium oxide in the range of 5-30% by weight is disclosed therein. Hirano et al. (U.S. Pat. No. 4,315,186) discloses a reflective incandescent display lamp with a front lens section fused to a reflective mirror section, the front lens section formed from a Nd.sub.2 O.sub.3 glass (preferably borosilicate as the base glass), the Nd.sub.2 O.sub.3 accounting for 0.5-5% by weight.
Another important rare-earth spectrum modifying element is cerium. One important use of cerium oxide incorporation in glasses is an increase in photo-darkening resistance when these glasses are exposed to ionizing radiation..sup.5 More important, cerium is the only rare-earth element that absorbs UV radiation while exhibiting no absorption in the visible region of the spectrum..sup.6 Therefore, cerium has value as a UV absorbing element when it is incorporated in a parent glass.
Traditionally, cerium has been used in combination with other rare-earth elements for decolorising glasses..sup.7 G. P. Drossbach patented a mixture of neodymium and cerium oxide for decolorization of glasses..sup.8 U.S. Pat. Nos. 3,865,747 (Greenberg) and 3,929,440 (Oldfield) disclose laser glasses containing neodymium and cerium. U.S. Pat. No. 3,685,747 (Greenberg) discloses that CeO.sub.2 in the range of 0.01-20 wt. % is added to ensure that uranium, another element in the glass composition, remains in its most fully oxidized state during the melting process; Nd.sub.2 O.sub.3 in the range of 0.01-30 wt. % is disclosed. U.S. Pat. No. 3,929,440 (Oldfield) discloses a lithium silicate laser glass containing neodymium and cerium; CeO.sub.2 in the range of 0-0.1 wt. % is added as a solarization agent; Nd.sub.2 O.sub.3 in the range of 2.5-6.5 wt. % is disclosed. U.S. Pat. Nos. 4,376,829 (Daiku), 4,390,637 (Daiku) and 4,521,524 (Yamashita) disclose glasses containing neodymium and cerium in combination for use in cathode ray tubes. U.S. Pat. No. 4,376,829 (Daiku) discloses a glass which absorbs in the yellow and blue regions of the spectrum for use in light-source cathode ray tubes; Nd.sub.2 O.sub.3 in the range of 1-10 wt. % and CeO.sub.2 in the range of 0.1-3 wt. % are disclosed. U.S. Pat. No. 4,390,637, (Daiku) discloses an X-ray absorbing and non-discoloring glass for use in color cathode ray tubes, said glass containing Nd.sub.2 O.sub.3 in the range of 0.1-5 wt. % and CeO.sub.2 in the range of 0-3 wt. %. U.S. Pat. No. 4,521,524 (Yamashita) discloses a glass filter for a CRT display, said glass containing Nd.sub.2 O.sub.3 in the range of 5-40 wt. % and CeO.sub.2 in the range of 0-6 wt. %; CeO.sub.2 added as a coloring agent.
It would be highly desirable to marry the benefits provided by the characteristic absorption spectra of neodymium, i.e., absorption of light in the yellow region of the visible spectrum, and cerium, i.e., absorption of UV radiation with no visible light absorption, into a family of glasses that are suitable for employment: as lamp envelopes, more particularly envelopes for high-temperature lamp (i.e., tungsten-halogen lamps), as well as sealed beam incandescent headlight lamps, whereby light sources of this type would emit minimum UV radiation and provide improved illumination in the surrounding environment. It is the basic purpose of the present invention to provide this desirable combination.